1. Field of the Invention
The present invention is directed generally to printing duplex and simplex copy sheets from electronic page information, especially suitable for low cost electrostatographic, ink jet, ionographic or other on-demand page printers with an endless duplex paper path loop. More particularly, the present invention relates to such printers which are integrated with on-line finishing devices.
2. Description of Related Art
The terminology "copiers", and "copies", as well as "printers" and "prints", is used alternatively herein. The terminology "imaging" and "marking" is used alternatively herein and refers to the entire process of putting an image (digital or analog source) onto paper. The image can then be permanently fixed to the paper by fusing, drying, or other means. It will be appreciated that the invention may apply to almost any system in which the images are made electronically, including electronic copiers.
Imaging systems (e.g., printers or copiers) typically include copy sheet paper paths through which copy sheets (e.g., plain paper) which are to receive an image are conveyed and imaged. The process of inserting copy sheets into the copy sheet paper path and controlling the movement of the copy sheets through the paper path to receive an image on one or both sides, is referred to as "scheduling". Copy sheets are printed by being passed through a copy sheet paper path (which includes a marking station) one or multiple times. Copy sheets which are printed on only one side (simplex copy sheets) in a single color usually pass through the copy sheet paper path a single time. Multipass printing is used to print images on both sides of a copy sheet (duplex printing), or to print a simplex sheet in multiple colors (one pass for each color). There are two general modes in which copy sheets to be multipass printed can be scheduled: "burst mode" and "interleave mode".
When scheduling in "burst mode", copy sheets are inserted into, imaged, and output from the copy sheet paper path without any "skipped pitches" existing between each consecutive copy sheet. A "pitch" is the portion (or length) of the copy sheet paper path in the process direction which is occupied by a copy sheet as it moves through the copy sheet paper path. A "skipped pitch" occurs when there is a space between two consecutively output copy sheets which is long enough to hold another copy sheet. Accordingly, when scheduling in "burst mode", copy sheets are output from the copy sheet paper path (and, thus, the imaging system) at a maximum rate because no skipped pitches exist between each consecutive copy sheet.
Various methods for scheduling copy sheets in "burst mode" are disclosed in, for example, the above incorporated U.S. Pat. No. 5,095,342.
When scheduling copy sheets in "interleave mode", skipped pitches are provided between each consecutively scheduled copy sheet. That is, a space is provided between each copy sheet inserted into and output from the copy sheet paper path. While other copy sheets may be eventually inserted in the space between two consecutively input sheets, these other sheets are inserted at a later time (described below) and are thus "interleaved" with the previously inserted copy sheets.
This "interleave mode" of copy sheet scheduling is typically employed in imaging systems which are capable of duplex printing (forming images on both sides of a copy sheet). Many imaging systems which are capable of duplex printing include copy sheet paper paths in the shape of a loop. The scheduling process involves: a) inserting a copy sheet into the loop; b) forming an image on a first side of the copy sheet at an imaging station; c) inverting the copy sheet (so that a second side of the copy sheet will face the imaging station when the copy sheet is reconveyed past the imaging station); d) forming an image on the second side of the copy sheet at the imaging station; and e) outputting the copy sheet from the paper path loop toward a final destination (a tray, a bindexer, finishing devices, etc.).
One reason why the "interleave mode" of scheduling is frequently used when duplex printing relates to the manner in which the original images are provided to the imaging station. For example, when the imaging system is using a recirculating document handler (RDH) to cycle a simplex document over a platen for exposure to a light source for forming duplex copies of the document, the imaging system exposes every other sheet in the simplex document so that a duplex copy of the document can be formed. For example, all even numbered pages in the document are exposed first to form a copy set consisting of copy sheets having even numbered pages on one side. Then, the odd numbered pages in the document are exposed, and these odd numbered pages are formed on the second side of the copy sheets containing the even numbered pages on side one.
The Xerox Corporation "9700" printer, duplex version schedules duplex copy sheets in an interleave mode of operation. It operates in essentially a trayless mode, with a long duplex loop path. Initially, prints (copies) of only the even sides are made, with one skip cycle (skipped pitch) between each print until the entire paper path is filled with even side prints alternated with skipped pitches. When the first completed even side (page 2) reaches the transfer area for the second side print (page 1), that page is printed on the back side. The next print to be made, however, is the next even side in the sequence printed on a blank sheet, and interleaved in the blank spaces (previously skipped pitches) left between sheets on the first pass. Thus, the job then proceeds at full productivity, intermixing (or interleaving) even sides printed on blank sheets for the first pass with odd sides printed on the back of previously completed even sides on their second pass. After the last even side is printed, the system resumes the skip pitch operation until the odd sides are printed on the last of the even side prints.
For a 30 page job, this "9700" printer duplex version page copying sequence can be represented as shown below. [Each "S" represents a skipped pitch. Previously printed sheet pages making their second pass for their second side copy are shown under the slash.]
First stage--[evens copied+skips=half productivity]: EQU 2, S, 4, S, 6, S, 8;
Second stage--[odds and evens intermixed--full productivity]:
______________________________________ 1/2, 10, 3/4, 12, 5/6, 14, 7/8, 16, 9/10, 18, 11/12, 20, 13/14, 22, 15/16, 24, 17/18, 26, 19/20, 28, 21/22, 30; ______________________________________
Third stage--[odds copied+skips=half productivity]; EQU 23/24, S, 25/26, S, 27/28, S, 29/30.
Note that with this "9700" printer sequence, 36 machine pitches are required to make 30 prints. So, for this 30 page job, the overall duplex operation is only 83% efficient. For longer jobs, the effective efficiency improves. But for shorter jobs the overall efficiency degrades, since there will still be 6 skipped pitches--"S".
The sequence used on Xerox Corporation "5700" printer is somewhat similar, except that it is not a trayless duplex loop system. All the completed first side sheets are stacked into a duplex buffer tray and later refed for side two printing. With this system, printer skip pitches are not required during the first stage of the job. The skip pitches are also not required for the third stage since the completed side ones can be fed at full thruput from the duplex tray. Thus, the "5700" duplexing is much more efficient than in the "9700". However, such duplex tray systems are inherently less reliable in some respects. The required duplex tray stacking, reseparating, and refeeding is implicated in the vast majority of duplex paper jams, and complicates job recovery. That is eliminated with the "9700" and other endless moving path duplex buffer loop systems.
Other conventional sequences for printers are also possible. For example, the Hewlett Packard HP "2000" uses a stack and re-feed method of duplex in which all even sides of the entire job are printed, followed by printing all of the odd sides. However, for this, the entire job (all the page images) must be stored in memory in order to insure jam recovery.
It is generally known that electronically inputted printers can desirably provide more flexibility in page sequencing (page, copying presentation order) than copiers with physical document sheet input. The printer input is electronically manipulatable electronic page media, rather than physical sheets of paper which are much more difficult to reorder or manipulate into a desired sequence. As also shown in the art noted hereinbelow, it is generally known that certain such reordered or hybrid document page copying orders or sequences may be copied onto a corresponding sequential train of copy sheets in an appropriate copier or printer to provide higher copying machine productivity yet correct page order copy output, especially for duplex copies made with a copier with trayless duplexing, i.e., providing a limited length endless buffer loop duplexing path for the copy sheets being duplexed.
Thus, electronically inputted imaging systems can operate in "burst mode" even when forming duplex copy sets. When operating in burst mode in an electronically inputted imaging system having an endless buffer loop duplexing paper path (no buffer tray), the duplexing paper path is completely filled with copy sheets (no skipped pitches) which are then imaged on both sides before being output from the duplexing paper path. Duplex burst mode scheduling causes duplex sheets to be output in small bursts of sheets (the duplex loop content) at full rated output.
However, it is becoming increasingly common to integrate on-line finishing devices with imaging systems. These on-line finishing devices directly receive copy sheets as they are output from the imaging system and perform various types of finishing operations on each copy sheet, or on each set of copy sheets. The finishing operations can be, for example: binding, stitching, folding, trimming, aligning, rotating, punching, drilling, slitting, perforating, and combinations thereof.
A problem which arises when integrating an existing finishing device with high speed imaging systems is that the finishing device may not be able to receive copy sheets at as high a frequency as the copy sheets can be output by the imaging system. For example, the imaging system described in U.S. Pat. No. 5,095,342 can output copy sheets at a rate of 135 per minute when operating in "burst mode". This rate is too fast for some finishing devices.
One example involves the use of the imaging system disclosed in U.S. Pat. No. 5,095,342 in connection with an on-line Signature Booklet Maker (SBM) to form signature booklets. A "signature" is a duplex printed copy sheet having two page images on each side. The signature sheet can be folded in half to form a booklet, or a plurality of signatures can be aligned, stitched together, and folded in half to form a multi-sheet booklet. A description of signature printing is provided in U.S. Pat. No. 4,727,042 to Smith, the disclosure of which is incorporated herein by reference. The device disclosed in U.S. Pat. No. 5,095,342 is capable of outputting signatures in bursts at a rate of 135 per minute, but the SBM may not be able to receive sheets at such a high rate.
The SBM can be constructed, for example, from variants of three existing finishing modules such as the AGR/Automatic Stitcher, the PA/Automatic Folder, and the TR/Automatic Trimmer, manufactured by C.P. Bourg for off-line use. All modules require mechanical modification to support front edge registration-vs. center registration and wiring modification to share basic signals with the printer. The printer exports sheet arrival times and end-of-set signals to the SBM equipment. The first module receives and aligns the copy sheets in a set (which set forms a single booklet) so that all sheets in the set are aligned with one another. The first module aligns each sheet by stopping the forward movement of the sheet (e.g., with a gate or sheet stop), and then laterally tapping each sheet against another sheet stop. Once all sheets in the set are received and aligned, the first module stitches or binds all the sheets in the set to each other at a central location (between each page image on each sheet). The stitching step can comprise, for example, stapling. Thus, the first module is referred to as a "saddle stitcher".
The stitched copy set is then forwarded to a second module which folds the stitched copy set in half about the stitch axis. Thus, the second module is referred to as a "folder".
The folded copy set is then forwarded to a third module where the edges of the sheets opposite from the fold are trimmed. Thus, the third module is referred to as a "trimmer". Trimming is necessary, particularly in large sets or booklets, because the edges of the sheets opposite from the fold become uneven after folding.
The time period required to receive and align each sheet in the first module is longer than the time period between the output of each consecutive copy sheet output by the imaging system when operating in "burst mode". Operating the imaging system in "interleave" mode would provide sheets to the SBM on every other pitch--an apparent half rate--thereby allowing adequate time for the SBM to register (receive and align) sheets.
However, using interleave mode scheduling for jobs not destined for the SBM would degrade system productivity by introducing "skipped pitches".
It is also possible to redesign the SBM to operate at a speed appropriate for handling sheets at the high burst mode rate. However, it is risky and expensive to redesign such machines. Additionally, if such machines are originally purchased from vendors, any warranties can be voided by such redesigning. Accordingly, there is a need for an imaging system which optimizes copy sheet output while compensating for differences in the operating rates of any finishing devices used therewith.
U.S. Pat. No. 4,466,733 to Pels, assigned to Xerox Corporation, discloses a method of duplex copying wherein the sequence of document and copy sheet feeding and copying is controlled to provide more efficient and rapid precollation copying. The method automatically selects one of two modes of duplex copying dependent upon the number of documents in a document set. In a formal duplex copying mode, the documents are copied only once in each copying circulation of the document set. Every other page of the document set is copied unidirectionally in reverse (N to 1) page order on only one side of the copy sheets. The copy sheets having been printed on one side are collected and stored in a buffer, forming a buffer set of copy sheets. Then, alternate documents are copied in proper sequence onto the opposite sides of the buffer set sheets to form sets of collated duplex copy sheets. If the number of documents in the document set is determined to be more than three but less than approximately ten, a higher productivity small document set copying mode is automatically selected. In the small document set copying mode, two identical buffer sets of alternate page documents are initially built and stored in the same buffer during two initial copy circulations of the document set. If the number of copy sets made is greater than two, all of the documents are copied in all intermediate document copying circulations to maintain the two buffer sets during the intermediate circulations by simultaneously rebuilding and depleting the two buffer sets. In the last two circulations alternate document pages are copied so that the two buffers are depleted and form sets of collated duplex copies.
U.S. Pat. No. 4,918,490 to Denis J. Stemmle (Xerox Corporation) discloses an endless duplex paper path loop having a single sheet inverter for inverting sheets in the duplex loop after side one imaging. Sheets are consecutively inserted into the duplex loop to avoid the first and third stage skipped pitches discussed above with reference to the "9700" system. Sheets are scheduled in 1-N order, with each multipage job set being electronically divided into consecutive batches, each batch containing a small number of pages equal to approximately twice the copy sheet length.
Also of interest is Mead Corporation U.S. Pat. No. 4,453,841 issued Jun. 12, 1984 to Bobick et al disclosing a trayless duplexing buffer loop path printer system, and noting particularly the page copy sequences shown in FIG. 6.
Some examples of other prior art copiers, and especially with control systems therefor, including operator console switch selection inputs, document sheet detecting switches, etc., are disclosed in U.S. Pat. Nos.: 4,054,380; 4,062,061; 4,076,408; 4,078,787; 4,099,860; 4,125,325; 4,132,401; 4,144,550; 4,158,500; 4,176,945; 4,179,215; 4,229,101; 4,278,344; 4,284,270; and 4,475,156. It is well known in this art, and in general, how to program and execute document handler and copier control functions and logic with conventional or simple software instructions for conventional microprocessors in a copier controller. This is taught by the above and other patents and various commercial copiers. Such software may vary depending on the particular function and particular microprocessor or microcomputer system utilized, of course, but will be available to or readily programmable by those skilled in the applicable arts without experimentation, from either descriptions or prior knowledge of the desired functions together with general knowledge in the general software and computer arts. It is also known that conventional or specified document and copy sheet handling functions and controls may be alternatively conventionally provided utilizing various other known or suitable logic or switching systems.
All references cited in this specification, and their references, are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.